Generically, both hot melt adhesives and curing urethane adhesives are well known generic adhesive classes. Hot melt adhesives can be conveniently applied by extruding the adhesive composition at elevated temperatures directly onto a work piece for the purpose of forming a structural bond with another work piece as the temperature of the adhesive compositional mass cools. While hot melt adhesives have many adhesive preparation and workpiece production benefits, hot melt adhesives have the drawback that after cooling to form a bond line, the thermoplastic compositions can be temperature sensitive. In other words, the bonding mass can lose bond strength as the temperature of the work piece and the bond line increase. Further, hot melt adhesives tend to be physically unstable in the presence of hydrocarbon solvents and some other organic compositions.
In sharp contrast, curing urethane adhesives have little green strength. After application of a urethane adhesive, the joined workpieces can require external mechanical support until the urethane adhesive can cure to a strong resilient highly crosslinked bondline. Cured polyurethane bonds have high tensile strength and have little or no temperature sensitivity. Cured urethanes can remain strong as temperatures rise until decomposition occurs, typically at very high temperatures. Such adhesives have value where initial green strength is not important since substantial bond formation requires a period of curing time which can range from a number of hours to a number of days.
Clearly an adhesive displaying both curing and hot melt properties in a single adhesive composition is a desirable goal since the resulting adhesive in theory could possess quick high strength green bond and strong crosslinked cured adhesive bonding.
One suggested adhesive is disclosed in Uchigaki et al, U.S. Pat. No. 3,931,077, which discloses reactive high viscosity hot melt adhesive compositions comprising a specific high viscosity reactive urethane prepolymer, a specific ethylene-vinyl acetate thermoplastic polymer and a phenol or abietic acid-type tackifying resin. Uchigaki teaches that hot melt moisture cure thermosetting adhesives should have a liquid urethane prepolymer having a viscosity higher than 300,000 cP at 25.degree. C. Uchigaki suggests that substantially lower viscosities result in adhesive compositions having a suitable viscosity for application but having inferior instant or green strength. Further, Uchigaki suggests using thermoplastic polymer compositions in the adhesive compositions such as an ethylene-vinyl acetate copolymer wherein the polymer contains ethylene in the range of about 90 to 70 wt-%. Further, Uchigaki suggests that concentrations of less than 70 wt-% ethylene results in adhesives having poor final adhesive strength. Further, Uchigaki teaches that tackifier components that can be used in the hot melt thermosetting adhesives should be a terpene phenol copolymer or an abietic acid type resin whose active hydrogens and double bonds are at least partly removed by esterification and/or hydrogenation. Such compositions include tackifier resins such as hydrogenated rosin, a hydrogenated rosin glycerine ester, a hydrogenated rosin pentaerythritol ester, disproportionated rosin, polymerized rosin, etc. Uchigaki further teaches that other types of tackifiers that are reactive with isocyanate groups on a prepolymer composition are undesirable. Further, Uchigaki suggests that some other types of tackifiers are insufficiently miscible with the adhesive components to result in a stable blend.
A further type of hot melt adhesive is disclosed in Reischle et al, U.S. Pat. No. 4,585,919 which teaches the combination of an isocyanate prepolymer, a thermoplastic polyurethane or polyester, and a synthetic resin selected from the group consisting of ketone resins, hydrogenation products of acetophenone condensation resins, and mixtures thereof.
In formulating reactive compositions, we have found substantial difficulty in obtaining compatible adhesive systems having a production viscosity that remain in a single phase when held at hot melt application temperatures. Many proposed adhesive formulations are not miscible and cannot be mixed under ordinary production conditions to form a stable adhesive mass. Further, even if miscible in production conditions involving severe mixing conditions, the compositions can phase and separate when held at the point of use in hot melt extrusion equipment. Further, certain phase stable mixtures can have poor machining characteristics. Accordingly, obtaining a compatible blend is critical.
We have also found that many curing adhesives when held at elevated temperatures in common application equipment, can suffer substantial rapid viscosity rise when held at elevated temperatures in production. Such a viscosity thermal instability can substantially reduce the value of the adhesives in the marketplace. Preferred adhesives can have stability in the viscosity increase over time such that the increase in viscosity over a 4 to 10 hour period is not substantially greater than about 25%.